Allyl complexes Grignard reagents

Allylvanadium complexes (see Allyl Complexes) are synthesized by allylation with Grignard reagents as shown in Scheme 25. Another synthetic route lies in the allylation of a vanadium anionic species with allylic chloride. Codimerization of vanadium-coordinated diphenylacetylene (tolane) and 1,3-butadiene affords the allyl complex (49) (Scheme 26). ... 

Cross-coupling techniques are always useful, especially when either high selectivity or novelty of product are achieved, as for example in the regioselective allylation of Grignard reagents by nickel and palladium phosphine complexes. It turns out that the regiochemistry of the product is entirely dependent on the catalyst used (Scheme 17). An adequate explanation for these observations must await further work. 

Allyl Complexes. Allyl complexes of uranium are known and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a uranium halide and an allyl grignard reagent. This synthetic method was utilized to obtain a rare example of a "naked" homoleptic allyl complex, U(T -C2H )4 [12701 -96-17, which decomposes at 0°C. Other examples, which are more stable than the homoleptic allyl complex have been synthesized, ie, U(allyl)2(OR)2 (R = alkyl), U(allyl)2X (X = halide), and U(allyl)(bipy)2. 

The reaction of 4-(phenylsulfonyl)azetidin-2-one (128) with nucleophiles such as dialkylcopper lithium and Grignard reagents gives 4-alkyl, 4-allyl, 4-vinyl or 4-ethynyl-azetidinon-2-one (129) in good yields (equation 99)83. The yields of several azetidin-2-ones obtained by this method are given in Table 10. The reaction apparently proceeds through an intermediate azetin-2-one 131 derived from the five-membered coordination complex (equation 100). 

Grignard reagents in the presence of a nickel complex to give both normal products and the products of allylic rearrangement. 

MgBr2-mediated asymmetric nucleophilic addition of Grignard reagents and allyl-tributyltin to aldehydes bearing sugar-derived jS- or y-tetrahydropyranyloxy chiral auxiliaries designed to complex with MgBt2 has been achieved. ... 

BCJ3785). Furthermore, 2-bromopyridine and related azaaromatics have been reported to react with allyl Grignard reagents to initially form complex 23 between the pyridyl nitrogen atom and the Grignard reagent. 

The Br/Mg-exchange reactions on alkenyl bromides are very sluggish. This problem was overcome by the use of 3-Bu2Mg LiCP . For example, Grignard reagent 66 is obtained by reaction of a-bromostyrene (67) with the complex 40 for 1 h at 25 °C. Quenching with benzaldehyde gives the allylic alcohol 68 in 93% yield (equation 41). 

Similar to Hoveyda s work, which describes NHC ligands for the allylic alkylation with dialkyIzinc reagents , Okamoto and coworkers reported the enantioselective allylic alkylation with Grignard reagents using an a-methyl naphthylamine-based NHC complex (Scheme 14) . 

Whereas THF and DME were suitable solvents, Lewis basic additives such as HMPA, DMSO or TMEDA hampered the reaction.82 These results point towards an intramolecular assistance of the zinc coordinated by the nitrogen atom. A related effect was evidenced in the additions of Grignard reagents to allylic alcohols83-87. Two mechanisms were proposed for the allylzincation of homoallylic amines involving formation of a complex with the... 

Failure for Grignard reagents to undergo similar additions to homoallylic amines may be explained by the greater stability of the complexes between an amine and zinc rather than magnesium. By contrast, addition of allylzinc bromide to allyl or homoallyl alcohols proceeded considerably less efficiently compared to amines and the regioselectivity was reversed (equation 55)81. 

The alcohol 10 looks like it might be formed by the addition of a Grignard reagent to an aldehyde. In fact, Patrick Steel of the University of Durham prepared 10 (Tetrahedron Lett. 44 9135,2003) by Diels-Alder addition of the transient silene derived from 7 to the diene 8. More highly substituted dienes lead to more complex arrays of stereogenic centers. The intermediate silacyclohexenes, exemplified by 9, should also engage in the other reactions of allyl silanes. 

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